1. Field of the Invention
The present invention relates to an apparatus and method for controlling operation of a reciprocating compressor, and in particular to an apparatus and a method for controlling operation of a reciprocating compressor which are capable of improving an operational efficiency of a compressor by making the operation frequency of the compressor according to the variation of a load consistent with a mechanical resonant frequency.
2. Description of the Background Art
These days various types of compressors are used, among them a reciprocating compressor is generally used. The reciprocating compressor sucks, compresses and discharges a refrigerating gas by a piston moving up and down inside a cylinder.
The reciprocating compressor is divided into a recipro type and a linear type according to the method of driving a piston.
The recipro type is such a type that changes the rotary movement of a motor into a linear movement and necessarily requires mechanical conversion devices such as a screw, a chain, a gear system, a timing belt, etc. for transforming a rotary movement into a linear movement. The use of the mechanical conversion device causes a large energy transformation loss and makes the structure of a compressor complex. Thus, in recent times, a reciprocating compressor employing a linear type in which a motor itself performs a linear movement is used.
In the reciprocating compressor employing the linear type, no mechanical conversion device is required because the motor itself directly generates a linear type driving force, the structure is not complex, a loss from an energy transformation can be reduced, and noise can be drastically reduced since there is joint regions generating friction or abrasion.
When the reciprocating compressor is used for a refrigerator or an air conditioner, a compression ratio can be varied by varying a stroke voltage applied to the reciprocating compressor, accordingly it is advantageous to a variable refrigerating capacity control.
FIG. 1 is a block diagram illustrating a construction of an apparatus for controlling operation of a reciprocating compressor in accordance with the prior art.
As depicted in FIG. 1, the prior art apparatus for controlling operation of a reciprocating compressor includes a reciprocating compressor 3 adjusting a refrigerating capacity by varying a stroke (a distance between a top dead center and a bottom dead center of the piston) by an up and down movement of a piston by a stroke voltage, a voltage detecting unit 5 detecting a voltage generated in the reciprocating compressor 3, a current detecting unit 6 detecting a current applied to the reciprocating compressor 3, a stroke calculating unit 4 estimating a stroke by using the detected current and voltage and a motor parameter, a comparator 10 comparing the calculated stroke with a certain stroke command value and outputting a comparison value according to the comparison result and a controller 2 controlling a stroke by varying the voltage applied to the motor based on the comparison value.
The control operation of the reciprocating compressor in accordance with the prior art will be described.
In the reciprocating compressor, when a stroke voltage is outputted by being inputted a certain stroke command value from a user, a stroke is varied by an up and down movement of a piston of a cylinder, a refrigerating gas inside the cylinder is transmitted to a condenser through a discharge valve, accordingly a refrigerating capacity can be adjusted. At this time, the voltage detecting unit 5 and the current detecting unit 6 detect a voltage and a current generated in the reciprocating compressor 3 and outputs them to the stroke calculating unit 4. The stroke calculating unit 4 utilizes the voltage, current and motor parameter to below equations and calculates a velocity of a piston (equation 1) and a calculated stroke (equation 2) by below equations.
                    Velocity        =                              V            M                    -          Ri          -                      L            ⁢                                          ⅆ                i                                            ⅆ                t                                                                        (                  Equation          ⁢                                          ⁢          1                )                                Stroke        =                              1            α                    ⁢                      ∫                                          (                Velocity                )                            ⁢                                                          ⁢                              ⅆ                t                                                                        (                  Equation          ⁢                                          ⁢          2                )            Here, α is a motor constant for calculating a stroke, i.e., a constant for converting an electric force into a mechanical force, R is a loss value due to resistance such as a copper loss or an iron loss, L is an inductance, and VM is a voltage between both ends of a motor.
The comparator 1 compares the stroke command value with the calculated stroke and applies a comparison signal to the controller 2, and the controller 2 varies the voltage applied to the motor of the reciprocating compressor 3 to control a stroke.
FIG. 2 is a flow chart illustrating a method for controlling operation of a reciprocating compressor in accordance with the prior art.
As depicted in FIG. 2, in the method for controlling operation of a reciprocating compressor, a voltage and a current are detected from the reciprocating compressor 3, and the calculation of a current calculated stroke is performed in the stroke calculating unit 4 (St1).
Afterwards, when the present stroke value calculated is smaller than the stroke command value, the controller 2 increases a stroke voltage (St2, St3), when the present stroke value calculated is larger than the stroke command value, the controller 2 decreases a stroke voltage (St2, St4).
However, even through a change of a mechanical resonant frequency occurs according to the variation of a load (for example, the outdoor temperature or a condenser temperature, etc.), the above-described prior art reciprocating compressor always controls a stroke at a constant operating frequency to drive the compressor, thus the operating efficiency is degraded.